1. Field of the Invention
The present invention relates to curable, flexible coating and paint compositions based on polyester resinous binder and containing pigment at a high pigment to binder ratio.
2. Description of Related Art
Coating formulations usually contain a number of components. A primary component is resin, which can be natural or synthetic. The resin acts as a polymeric coating binder, or as a polymeric coating vehicle for the coating formulation. In addition, most coatings require a solvent, and the coating may also contain a wide variety of additives. Further, many coatings also contain a crosslinking agent, which after application of the coating vehicle to a substrate, reacts chemically with the resin during a curing stage to produce a film containing a crosslinked network. The crosslinked network is often necessary for the production of good film properties. The curing stage can be conducted at ambient conditions ("air-dry system") or at elevated temperatures ("baked system"). In either case, the solvent is evaporated during the curing stage, resulting in a coating film. A number of properties are important for the coating film, including hardness, flexibility, weather resistance (weatherability), chemical resistance, solvent resistance, corrosion resistance, adhesion to various substrates, and impact resistance. The properties depend on many factors including type, molecular weight, monomer composition, and glass transition temperature (Tg) of the resin; type and amount of the crosslinker; curing conditions; curing catalyst; and additives. Variations of these parameters can be used to create a wide range of differences in film properties to fit requirements for a number of diverse applications. However, it is not always possible to optimize all of the desirable properties simultaneously.
Coil coating is a well known process for coating metallic substrates. In this process, metal in rolled sheet form is unrolled and fed to a coating zone wherein a suitable curable coating composition is applied by any suitable technique such as spray coating or dip coating. The coated sheet is then fed into a dryer zone wherein sufficient heat is applied to dry the coating and cure the coating formulation. The coated sheet is then rewound and transferred for further handling and fabrication into such articles as bottle caps, containers, labels, emblems, building panels, siding for buildings and the like.
Several criteria must be met by curable coatings which are suited for coil coating applications. They must be rapid curing since coil coating lines often operate at a sheet speed of 200 ft./minute or higher. The cured coating must be of sufficient hardness to resist abuse associated with the forming or stamping of shaped articles therefrom, as well as the weight of the coil pressing on its inner layers when rolled up. The coatings must at the same time be flexible enough to be rolled up, unrolled and withstand the forming and stamping operations without cracking or chipping. In many cases, the coatings must also exhibit good weather resistance, especially for outdoor applications. And the coatings should be formulated with sufficient pigment content to provide optimum hiding power over the underlying metal surface, thereby yielding coated surfaces which are more appealing to the eye and less expensive to formulate.
There are a number of coating formulations disclosed in the art based on curable polyester binder material which are disclosed to be suitable for use in coil coating applications. Representative examples of such formulations are disclosed in U.S. Pat. Nos. 3,714,090, 4,140,729, 4,229,555, 4,393,121, 4,520,188 and 4,734,467. These formulations generally comprise a high solid content solution of curable polyester resin in organic solvent, which solution also contains a crosslinking agent such as an amino or aminoplast crosslinker, an optional crosslinking catalyst and a pigment and/or extender dispersed in the solution. Where present, the pigment content to resin content generally is at a weight ratio of less than 1.25:1 respectively. These formulations may also contain other conventional ingredients used in such formulations.
The weight ratio of pigment to binder (P/B) is generally less than 1.25:1, usually within the range of 0.8:1 to 1.1:1. This P/B ratio is determined to be the best in terms of optimum properties of the final film and attempts to raise the P/B to 1.5 or 2.0 or higher result in the deterioration of some or many of the important coating properties. The properties which are adversely effected may include flexibility, solvent resistance or weatherability. Thus, the advantages that certain polyester-based coating formulations may offer are lost at higher pigment loadings, thereby rendering such materials less suitable or unsuitable for such applications.